A variable gain amplifier is widely applied in a communication circuit or an audio/video analog signal processing circuit. Generally, a variable gain amplifier needs to have a variable gain, with its output current in an exponential proportion to its control voltage. When a decibel value is used as a scale, a gain curve is a straight line, generally called a linear-in-dB relationship (Linear-in-dB). In addition, a variable gain amplifier needs to have a large output gain range, a large output dynamic range, low noise, a low voltage, and low power consumption. Performance of a variable gain amplifier directly affects system performance. For a variable gain amplifier, however, there are substantial and high requirements for its indicators, making it rather difficult to design a variable gain amplifier with good performance.
The inventor has found, in the process of implementing the present invention, that a linear-in-dB relationship between an output current and a control voltage of a variable gain amplifier in the prior art is not ideal and cannot well meet a requirement for a variable gain amplifier in practical use.